188 DR. F. .7. NORTH ON [vol', lxxvi. 



In the absence of specimens and accurate stratigraphieal evidence 

 a detailed discussion of this interesting subject is not possible, and 

 the following summary of the evolution of Syringothyris is to a 

 great extent based upon the descriptions given bv James Hall & 

 J. M. Clarke, 1 G. H. Girty, 3 and H. S. Williams.* 



In many Lower Devonian Spirifers there was, as the organism 

 reached maturity, a tendency to deposit shelly matter (an apical 

 callosity) in the beak of the pedicle-valve. From the free, that is, 

 dorsally-directed, end of that callosity, a plate (the transverse 

 plate) extended forward between the delthyrial supporting-plates, 

 anterior to, or below, the deltidium. Originally an accompaniment 

 of mature growth, these characters seem eventually to have become 

 permanent, and in late Devonian and Carboniferous times were 

 essential features in many species. 



In some forms, in which the radial ornament extended over the 

 whole shell, the plate remained unchanged, while in others, charac- 

 terized by a smooth fold and sinus, it became curiously modified. 4 

 A pair of longitudinal ridges were developed on the nether surface 

 of the plate, and these, in the course of time, gave rise to two 

 lamellse, the free ends of which curled one towards the other to 

 form an incomplete tube or syrinx. The formation of the syrinx 

 was preceded and accompanied by a flattening and great elevation 

 of the cardinal area of the valve. 



A departure from the usual conditions is seen .in the species 

 described by Weller as Pseudosyrinx sampsoni. 



Pseudostrinx Weller. 



' The Mississippian Brachiopoda of the Mississippi Valley Basin' Illinois Geol. 

 Surv. Monogr. 1 (1914) pp. 401-405. 



The forms referred to this genus resemble Syringotliyris in all 

 respects save one : namely, the nature of the transverse plate. In 

 shape and size, in the absence of plications on the fold and sinus, 

 in the differentiation of the area into a central and two lateral 

 portions (of which only the former is marked by vertical striations), 

 and in the punctation of the shell-substance, Pseudosyrinx agrees 

 with Syringotliyris ; but the transverse plate is quite simple, and. 

 bears no trace whatever of the existence of a syrinx. The possi- 

 bility that the absence of the syrinx might be due to the mode of 

 preservation of the shell was carefully considered by the author of 

 the genus, who found that the feature was of constant occurrence 

 in specimens preserved in such a way as to show with certainty 

 that no syrinx had ever existed. 



Pseudosyrinx is found in the Lower Mississippian of North 

 America : that is, it was contemporaneous with Syringothyris ; 

 and the coexistence of forms, externally indistinguishable, the 



1 Hall & Clarke [14]. 2 Girty [12] p. 51. 



3 Williams [37] pp. 61-62. 



4 These two types practically coincide with two of the groups into which 

 the Spirifers were divided by Hall & Clarke : namely, the ' aperturati ' and the 

 ' ostiolati ' respectively. 



